Grid square TF. The map shows
The WashThe Wash and the North Sea, as well as
places within the counties of Lincolnshire, Cambridgeshire and
Norfolk.

The grid is based on the
OSGB36OSGB36 datum (
Ordnance SurveyOrdnance Survey Great Britain
1936,[citation needed] based on the Airy 1830 ellipsoid),[citation
needed] and was introduced after the retriangulation of
1936–1962.[citation needed] It replaced the previously used Cassini
Grid which,[citation needed] up to the end of World War Two, had been
issued only to the military.[citation needed]
The Airy ellipsoid is a regional best fit for Britain; more modern
mapping tends to use the GRS80 ellipsoid used by the Global
Positioning System (the Airy ellipsoid assumes the Earth to be about
1 km smaller in diameter than the GRS80 ellipsoid, and to be
slightly less flattened). The British maps adopt a Transverse Mercator
projection with an origin (the "true" origin) at 49° N,
2° W (an offshore point in the
English ChannelEnglish Channel which lies
between the island of
JerseyJersey and the French port of St. Malo).[3] Over
the Airy ellipsoid a straight line grid, the National Grid, is placed
with a new false origin to eliminate negative numbers, creating a
700 km by 1300 km grid. This false origin is located
south-west of the Isles of Scilly. The distortion created between the
OS grid and the projection is countered by a scale factor in the
longitude to create two lines of longitude with zero distortion rather
than one. Grid north and true north are only aligned on the
400 km easting of the grid which is 2° W (OSGB36) and
approx. 2° 0′ 5″ W (WGS 84).
OSGB 36 was also used by
AdmiraltyAdmiralty nautical charts until 2000[citation
needed] after which WGS 84 has been used.
A geodetic transformation between OSGB 36 and other terrestrial
reference systems (like ITRF2000, ETRS89, or WGS 84) can become quite
tedious if attempted manually. The most common transformation is
called the Helmert datum transformation, which results in a typical
7 m error from true. The definitive transformation from ETRS89
that is published by the OSGB is called the National Grid
Transformation OSTN02.[4] This models the detailed distortions in the
1936–1962 retriangulation, and achieves backwards compatibility in
grid coordinates to sub-metre accuracy.
Datum shift between OSGB 36 and WGS 84[edit]
The difference between the coordinates on different datums varies from
place to place. The longitude and latitude positions on OSGB 36 are
the same as for WGS 84 at a point in the Atlantic Ocean well to the
west of Great Britain. In Cornwall, the WGS 84 longitude lines are
about 70 metres east of their OSGB 36 equivalents, this value rising
gradually to about 120 m east on the east coast of East Anglia.
The WGS 84 latitude lines are about 70 m south of the OSGB 36
lines in South Cornwall, the difference diminishing to zero in the
Scottish Borders, and then increasing to about 50 m north on the
north coast of Scotland. (If the lines are further east, then the
longitude value of any given point is further west. Similarly, if the
lines are further south, the values will give the point a more
northerly latitude.) The smallest datum shift is on the west coast of
ScotlandScotland and the greatest in Kent.
Datum shift between OSGB 36 and ED 50[edit]
These two datums are not both in general use in any one place, but for
a point in the
English ChannelEnglish Channel halfway between
DoverDover and Calais, the
ED50ED50 longitude lines are about 20 m east of the OSGB36
equivalents, and the
ED50ED50 latitude lines are about 150 m south of
the
OSGB36OSGB36 ones.[citation needed]

For the first letter, the grid is divided into squares of size
500 km by 500 km, outlined in dark grey on the map to the
right. There are four of these which contain significant land area
within Great Britain: S, T, N and H. The O square contains a tiny area
of North Yorkshire, almost all of which lies below mean high tide.[5]
For the second letter, each 500 km square is subdivided into 25
squares of size 100 km by 100 km, each with a letter code
from A to Z (omitting I) starting with A in the north-west corner to Z
in the south-east corner. These squares are outlined in light grey on
the map, with those containing land lettered. The central (2° W)
meridian is shown in red.
Grid digits[edit]
Within each square, eastings and northings from the south west corner
of the square are given numerically. For example, NH0325 means a
1 km square whose south-west corner is 3 km east and
25 km north from the south-west corner of square NH. A location
can be indicated to varying resolutions numerically, usually from two
digits in each coordinate (for a 1 km square) through to five
(for a 1 m square); in each case the first half of the digits is
for the first coordinate and the second half for the other. The most
common usage is the six figure grid reference, employing three digits
in each coordinate to determine a 100 m square. For example, the
grid reference of the 100 m square containing the summit of Ben
Nevis is NN 166 712. (Grid references may be written with or without
spaces; e.g., also NN166712.) NN has an easting of 200 km and
northing of 700 km, so the
OSGB36OSGB36 National Grid location for Ben
Nevis is at 216600, 771200.
All-numeric grid references[edit]
Grid references may also be quoted as a pair of numbers: eastings then
northings in metres, measured from the southwest corner of the SV
square. Note that 14 digits may be required for locations in Orkney
and further north. For example, the grid reference for
Sullom VoeSullom Voe oil
terminal in the Shetland Islands may be given as HU396753 or
439668,1175316.
Another, distinct, form of all-numeric grid reference is an
abbreviated alphanumeric reference where the letters are simply
omitted, e.g. 166712 for the summit of Ben Nevis. Unlike the numeric
references described above, this abbreviated grid reference is
incomplete; it gives the location relative to an OS
100×100 km square, but does not specify which square. It is
often used informally when the context identifies the OS 2-letter
square. For example, within the context of a location known to be on
OS Landranger sheet 41 (which extends from NN000500 in the south-west
to NN400900 in the north-east) the abbreviated grid reference 166712
is equivalent to NN166712. If working with more than one Landranger
sheet, this may also be given as 41/166712.
Alternatively, sometimes numbers instead of the two-letter
combinations are used for the 100×100 km squares. The numbering
follows a grid index where the tens denote the progress from West to
East and the units from South to North. In the north of Scotland, the
numbering is modified: the 100 km square to the north of 39 is
numbered N30; the square to the north of 49 is N40, etc.
Summary parameters of the coordinate system[edit]